Re: who noticed
Fillers are non-canon episodes(does not happen in the manga). Usually, filler episodes are introduced when the anime is getting too close to the actual manga and they go on for the time that is needed for the manga to go further-on and dig a decent gap between anime and manga. Althought, sometime, the filler episodes do last for quite a long time and in most cases(well in naruto's case anyways) aren't that interesting.
Some may disagree on that last sentence, but hey, it's my point of view.
However, some episodes that could be considered fillers are infact sort of in the story(Kakashi gaiden for example) but introduced rather late.
In kakashi gaiden's case, it was introduced in the story as some sort of transition from part I to II and thus it was regarded as a special "side" story(you will see what I mean soon), but since it was animated during part II(episode 119-120), it can be considered filler to the "shippûden" era(althought, it was one damn good filler ^^).
Anyways, fillers are ,as said before, special episode that are "needed" to let the manga go further on and thus aren't canon in any ways.